DIRAC experiment (PS212)

1. DIRAC experiment (PS212) A.Benelli JINR, Zurich University MESON 2012 Conference, Krakov, June, 2012.

2. DIRAC collaboration CERN Tokyo Metropolitan University Czech Technical University IFIN-HH Institute of Physics ASCR JINR Nuclear Physics Institute ASCR SINP of Moscow State University INFN-Laboratori Nazionali di Frascati IHEP University of Messina Santiago de Compostela University KEK Bern University Kyoto University Zurich University Kyoto Sangyou University

3. ChPT predicts s-wave scattering lengths: Kp : 1-loop approx. a1/2= 0.19 + 0.02 a3/2 = -0.05 + 0.02 a1/2- a3/2 = 0.238+-0.002 P. Büttiker et al. 2004 2-loop approx. a1/2- a3/2 = 0.267 pp : a0 = 0.220 + 0.005 a2 = -0.0444 + 0.0010 a0 – a2 = 0.265 + 0.004 V.Bernard, N. Kaiser, U. Meissner 1991 G. Colangeloet al., Nucl. Phys. B 603 (2001) 125 J. Bijnens, P.P. Donthe, P.Talavera 2004 t = (2.9 + 0.1) fs Roy-Steiner equations : a1/2- a3/2= 0.269 + 0.015 P. Buttiker et al 2004 t = (3.7 + 0.4) fs

4. The main DIRAC aim is the accurate measurement of pp scattering lenghts and the first measurement of pk scattering lenghts • App lifetime measurement • observation of Kp atoms and their lifetime measurement • long lived atoms observation

5. DIRAC setup Upgraded DIRAC setup MDC - microdrift gas chambers, SFD - scintillating fiber detector, IH – ionization hodoscope. DC - drift chambers , VH – vertical hodoscopes, HH – horizontal hodoscopes, Ch – nitrogen Cherenkov , PSh - preshower detectors , Mu - muon detectors Modifided parts

6. PS magnet target MDC SFD IH p (24 GeV) DCs target VH HH π+ A2π - (nA) p (NA) π−

7. π+ (NCC) p π− Coulomb pairs ,,,… π+ (NCC) π− p Coulomb pairs π+ η, η’,… (NNC) π+ π0 p Non-Coulomb pairs π− η, η’,… π+ p π0 π+ p (NACC) π− p Accidental pairs K p X

8. (2003: GEM-MSGC) TRACKING Scintillation fibres detector Micro Drift Chambers 18 planes : X, Y, U Area: 80x80 mm Gas mixture: Ar(0.33)+ iC4H10(0.66)+H2O(0.01) Anode pitch: 2.5 mm 32 wires in a plane Sell size: 2.5x2 mm Drift time: 26 ns Time resolution < 1ns Space resolution < 80mm 2 tracks resolution <200mm Read-out time < 3ms Drift chambers DC1:2x80x40 cm DC2: X,Y 80x40 cm DC3: X,Y 112x40 cm DC4: X,Y,X,Y 128x40cm Both arms : 2016 ch Anode pitch: 10mm Cell:10x10cm Cathode 20 mm carbon-coated mylar Anode wires: 50 mm Copper-beryllium alloy Drift velocity: 50 mm/ns Amplitude: 1 mA Pulse width: 20 ns Resolution: 90 mm IonisationHodoscope

9. Vertical Hodoscope Horizontal Hodoscope Trigger requests tracks coplanarity

10. nA = 21227 + 407

11. Published results on π π atom: lifetime & scattering length * theoretical uncertainty included in systematic error

12. pp and kp geometrical acceptance p p k p

13. Cherenkov detectors Heavy Gas Nitrogen Counter Aerogel (1.008)p>1.1GeV (1.015)p>0.9GeV

14. Heavy Gas Cerenkov detector Pion response Electron response Light vs momentum

15. Aerogel Cerenkov detector Aerogel n=1.008 module n=1.015 module protons kaons

17. nApk= 173+54 3.2 s t > 0.8 fs 90% CL

18. Preshower detector Cut efficiency : 12.5% e+e- 97.5% pp e+e- subtraction 97.8% pp

19. π-K+and π+K- prompt pairs Run 2008-2010, statistics with low and medium background (⅔ of all statistics). Point-like production of all particles. The e+e‒ background was not subtracted. Coulomb pairs non-Coulomb pairs Atomic pairs 5.3s 26% relative error on |a1/2-a3/2|

20. π+π‒ prompt pairs Coulomb pairs Coulomb pairs non-Coulomb pairs non-Coulomb pairs Atomic pairs Atomic pairs Run 2008-2010, statistics with low and medium background (⅔ of all statistics). Point-like production of all particles. The e+e‒ background was not subtracted.

21. Long lived atoms In order to measure an other combination of scattering length : 2a0+a2 2011-2012 data taking App & CC and NC * The A2πdecay in the p-state is forbidden by angular momentum conservation. The lifetime of the A2πatom in the 2p state (τ2p=1.17 ·10-11 s) is determined by the 2p–1sradiative transition with a subsequent annihilation from1s state (τ1s=3 ·10-15 s): π+ + π- π0 + π0

22. 2010-2011 : Production of A2p in Beryllium target Distribution over |QL| of p+p- pairs collected in 2010-2011 with Beryllium target with the cut QT<1 MeV/c. Experimental data (points with errors) have been fitted by a sum of simulated CC and NC pairs (dashed line). Produced atoms normalized on the proton flux : NA2p = (5.1 + 0.5) x 10 -14

23. Simulation of the permanent magnet at the target Simulated atomic pairs from long lived atoms(light area) over QY above the background of pp pairs produced in the Beryllium target with cuts |QX, QL|<1MeV/c (hatched area). On the left side without the magnet and on the right with the magnet used during 2011. Atomic pairs from long lived atoms y x z

24. 2011 data : Experimental distribution of e+e- over QY 2 1 Pairs generated on Be target before magnet 2 1 Pairs generated after magnet

25. Magnet for 2012 run • 2 new magnets: Sm2Co17, high resistivity against radiation, B=0.26T, expected signal > 9 sigma. • New retracting device allows to replace magnet fast. BLUE … magnet yoke GREY … magnet poles RED … magnet shimming PURPLE … Pt foil

26. Energy splitting in theory Electromagnetic corr. DEem2s-2p = -0.012 eV The observation of ππ atom long-lived states opens the future possibility to measure the energy difference between ns and np states DE(ns-np) and the value of ππ scattering lengths |2a0+a2|.

27. DE in practice .. observation method The lifetime of the np-states is about 103 larger than the ns-states, so it is possible to measure the energy difference of these levels by exerting an electric field (Stark effect)on the atom and tracking the field dependence of the decay probability. The influence of an magnetic field on the A2π atom lifetime opens the possibility to measure the splitting between 2s and 2p levels. Pt foil

28. SPS (450 GeV) : yield of A2p and Akp will increase of a factor 20 per proton-nucleus interaction.

29. Thank you for your attention